a 5AR4 after a diode bridge? what is the purpose?

[jarthel]

I saw the circuit from this: http://www.simpletube.com/databank/S...7_6SN7_10K.gif

thank you.

[rcavictim]

Gives the B+ a delayed onset so the rest of the tubes can reach cathode emission before they get hit with full B+. This can be done with half the SS diodes however in what is called a hybrid bridge, except the circuit shown, with both diodes of the 5AR4 in parallel would offer more B+ current capability.

I like the idea of using a SS bridge followed by a slow warm-up TV damper diode rather than a 5AR4, 5U4, etc. Dampers can have less internal voltage drop.

[tjl]

Agree to "rcavictim" mention about 5AR4 function as slow warm-up and increase cathode output current slowly,

It is better than using timer circuit by a relay contact suddenly rising to high B+ voltage.

I like to use silicon diode rectify and following by TV damping tube too,

TV damping tube have less internal impedance than rectify tubes,

If at full load maximun current output condition 5U4 will drop almost 58 V but some type of TV damping tube was less than 20 V droping only!

[pchw]

Originally posted by rcavictim
Gives the B+ a delayed onset so the rest of the tubes can reach cathode emission before they get hit with full B+. This can be done with half the SS diodes however in what is called a hybrid bridge, except the circuit shown, with both diodes of the 5AR4 in parallel would offer more B+ current capability.

I like the idea of using a SS bridge followed by a slow warm-up TV damper diode rather than a 5AR4, 5U4, etc. Dampers can have less internal voltage drop.

Hi,

Do you mean something like 6W4GT and 6AX4GT? I have been wondering what to do with these half-wave rectifiers in my box ......

Thanks!!

[rcavictim]

Originally posted by pchw


Hi,

Do you mean something like 6W4GT and 6AX4GT? I have been wondering what to do with these half-wave rectifiers in my box ......

Thanks!!

Yes Fred, those tubes. They can be parallelled too for higher current demands and increasing the tube count which always looks cool.

They usually operate on 6.3 volts (but there are 12AX4`s out there) and have a high heater to cathode insulation rating. Despite this I`d feel safer giving them their own dedicated heater winding on the power xfmer, or a separate xfmer.

[pchw]

Thank you for the info, Bob.

A few more stupid questions:
1. I looked at the specs of the 6W6GT and 6AX4GT, they both specify the voltage drop at 250ma. Can I assume that the drop will be lowered if the current draw is lesser?

2. None of the data sheet says anything about the max of the filter cap after the diode, can I assume the no more than 40uF after a tube rectifier rule?

3. Let say I have
250VAC trafo -> rectifier bridge -> 6AX4GTB -> 1st CAP (40uF?) -> 8H Choke -> 390uF CAP -> 40ma load
What kind of voltage I expect at 1st CAP?

Thanks!!!

[rcavictim]

Fred,

The thermionic rectifier tube looks like a resistor, so the lower the current pulled through unit the lower the voltage drop across it.

You need to download a free copy of PSUD (power supply designer). It is free for use by hobbyists. There is also a list of popular rectifiers, both ss and tube to load into the program and an updated chart of added rectifier type parameters. This program even gives you charts predicting the voltages and currents in the circuit from turn-on until the thing stabilizes. Very cool. This program will predict and graph peak rectifier currents so as to answer your second question in regards to first filter cap size.

My intuition tells me that because you have a bridge doing the rectification before the tube diode, the fact that the tube now receives bouncy DC at twice the normal ripple frequency it will probably handle a larger than the 40 uF cap stated by the general rule of thumb.

http://www.duncanamps.com/psud2/download.html